This grant will investigate the hypothesis that sphingolipids are mediators of environmentally-induced toxicity via their participation in signal transduction pathways that regulate gene transcription, growth and apoptosis. Two components of this hypothesis will be tested using benzo[a]pyrene (BP) as a prototypic toxicant of polycyclic hydrocarbons and similar compounds: 1) that BP disrupts normal sphingolipid metabolism and signaling, which contributes to (and possibly accounts for) its cytotoxicity; and, 2) that the disruption of normal sphingolipid metabolism and signaling contributes to reduction of downstream biochemical processes important in detoxification of xenobiotics and maintenance of cellular integrity. The model system that has been selected for these studies is a human intestinal cell line, HT29 cells, for which BP is cytotoxic and in which preliminary results demonstrate an association between its toxicity and sphingolipid metabolism. The specific aims of this grant are: 1) to characterize the mechanisms whereby BP alters sphingolipid metabolism, and to identify the role of these metabolites in the effects of BP on HT29 cells. 2) To identify the down-stream cell signaling events (e.g., activation of nucleoproteins, AP-1, stress-activated kinases, induction of phospho retinoblastoma protein dephosphorylation, etc. ) that are involved in the effects of BP. And, 3) to evaluate the efficacy of sphingolipids, sphingolipid analogs, and inhibitors of sphingolipid metabolism as in the protection of cells against the effects of BP. The results of these experiments will provide fundamental information about the importance of sphingolipid signaling in the action of toxicants such as benzo[a]pyrene, and will lead to novel interventions in preventing the cellular dysfunction and disease they produce.